Daphne Caruana Galizia, who led an investigation of corruption in Malta, was killed by a car bomb.
Photograph: Matthew Mirabelli/AFP/Getty Images

The heads of eight of the world’s largest news organisations have called for the European commission to investigate the murder of Maltese journalist Daphne Caruana Galizia and the independence of the media in Malta.

The editors and directors have written a letter to Frans Timmermans, the first vice-president of the commission, which describes the murder of Caruana Galizia as “shocking” and an “appalling reminder” of the dangers that journalists and citizens practising journalism face as they try to uncover corruption and criminal behaviour.

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The signatories include Katharine Viner, the editor-in-chief of the Guardian, Dean Baquet, executive editor of the New York Times, Jérôme Fenoglio, director of Le Monde, and James Harding, the director of news and current affairs at the BBC.

Caruana Galizia was killed last month by a car bomb near her home. She had led an investigation into corruption in Malta using the Panama Papers – with the government and the opposition implicated – and the readership for her blogposts often exceeded the circulation of the country’s newspapers.

No group or individual has come forward to claim responsibility for the attack. Thousands of people attended a rally after Caruana Galizia’s death demanding justice for the journalist. Her son said the island was a “mafia state”.

The editors and directors say the murder cannot be allowed to achieve the “clear objective of silencing her investigation into corruption at the highest levels in Malta”. They also point to analysis by the European commission that raised concerns about the lack of political independence of the Maltese media and that it was the “only EU country that has such extensive media ownership by the political parties”.

Other editors to sign the letter are Wolfgang Krach of Süddeutsche Zeitung, Lionel Barber of the Financial Times, Mario Calabresi at La Repubblica and Antonio Caño, of El País.

The letter to Timmermans adds: “Daphne’s murder, combined with the structural issues the commission identified, demonstrate the need for a full investigation into the state of media independence in Malta by the commission.

“We ask that you use your office to engage the Maltese government in urgent dialogue to ensure that it is aware of its obligations as a member of the European Union to uphold the rule of law, and to maintain press freedom and free expression.

“The murder of Daphne Caruana Galizia demonstrates the danger that journalists face in the pursuit of truth. It also demonstrates the fear that the corrupt and powerful have of being exposed.

“We request that you use all powers at your disposal to ensure that Daphne’s death is fully investigated, and to send a clear signal of support to journalists working in the public interest, in Malta and all over the world.”

In response to the letter, Timmermans called on the Maltese authorities to “leave no stone unturned” in their investigation.

He said: “For the European commission there can be neither real democracy nor rule of law without free media. This is what we said clearly last week when the European parliament discussed the barbarous assassination of Daphne Caruana Galizia. Media freedom is not just a value per se, it also underpins all the other values we hold dear. If journalists are silenced, so is democracy.

“We insist that the Maltese authorities leave no stone unturned to make sure that this atrocious, barbarous assassination does not lead to the situation that the perpetrators apparently want to achieve: that no one dares ask pertinent questions and no journalist dares investigate the powers that be.

“The eyes of Europe are on the Maltese authorities. Answers need to be provided and crimes prosecuted. We want those directly and indirectly responsible for this horrible murder to be brought to justice. And we want the investigations to run their full course, so that any other related wrongdoings that may emerge can also be prosecuted and potential structural problems be resolved.

“Tomorrow the flags at the commission HQ will fly half-mast in honour of Daphne Caruana Galizia and all those who have given their lives for the freedom of speech without which freedom is an empty shell.”